Deviated wellbore drilling system and apparatus
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to improvements in well drilling system and apparatus, and more particularly to downhole drilling motor apparatus having a bent or curved drive section. 
     2. Brief Description of the Prior Art 
     It has been recognized that a number of advantages can be gained in drilling wells by employing a stationary drill pipe or drill string which has attached at its lower end a downhole motor, the drive section of which is connected to and rotates a drill bit. In such apparatus a fluid, e.g., air, foam, or a relatively incompressible liquid, is forced down the stationary drill pipe or drill string and on passing through the fluid-operated motor causes rotation of a shaft ultimately connected to the drilling bit. The drill string is held or suspended in such a manner that it does not rotate and therefore may be regarded as stationary. However, it is lowered in the well as drilling proceeds. 
     The forces required to rotate the rotary bit at the bottom of the string are such that in the usual situation the fluid operated motor must be quite lengthy. Conventional straight hole drilling motors such as the Moineau (Moyno) motor comprise three sections, the rotor/stator section which contains a rubber stator and a steel rotor; the universal section which contains the universal joint or flexible connection that converts the orbiting motion of the rotor to the concentric rotary motion of the bit; and the bearing pack section which contains radial and thrust bearings to absorb the high loads applied to the drill bit. The rotor/stator section of the motor is typically 2-3 times longer than the bearing pack section. 
     In directional drilling, drilling motors of this general character are utilized wherein a bend may be located in the drill string above the motor, a bend may be placed in the motor housing below the rotor/stator drive section, or the bit or output shaft is angularly offset relative to the drive section axis. 
     In some directional drilling systems, such as Dellinger al, U.S. Pat. No. 4,577,701 and British Patent 1,494,273, the practice has been to position a &#34;bent sub&#34; between the top of the fluid-operated motor and the axis of rotation of the bit to the axis of the drill pipe. However, due to the length of the motor required and other structure connecting the rotor of the motor to the bit, the spacing of the &#34;bent sub&#34; from the bit is excessive. This distance frequently amounts to approximately 22 feet or more which is objectionable due to the fact that it is difficult to position and to maintain the orientation of the bit in relation to the axis of the drill pipe. 
     In an attempt to overcome this problem other systems have been designed to place the bend closer to the bit, such as Nielson et al, U.S. Pat. No. 3,260,318. 
     These types of systems modify the universal section of the drilling motor. Because the lower end of the rotor in the aforementioned types of motors gyrate about the axis of its stator, some form of universal joint or flexible connection is employed in the driving connection between the rotor and the bit which rotates about a stationary axis. As a clearance must exist between this universal joint or flexible connection and the walls of the surrounding housing to accommodate the flexibility of movement, a bend is formed in the housing of the universal section of the motor. In this manner, the axis of rotation of the bit is angularly related not only to the axis of the drill string but also the axis of the fluid-operated motor. This aids in obtaining and maintaining control and orientation of the bit. However, placing the bend in the housing surrounding the universal joint limits the severity of the bend which can be used. 
     Other systems, such as Henderson, U.S. Pat. No. 3,667,556 and Kamp, European Patent 109,699 disclose apparatus wherein the drill bit or output shaft is angularly offset relative to the motor drive section axis. Applicant&#39;s prior patent having the same assignee as this application, Maurer et al U.S. Pat. No. 4,823,053 shows an apparatus substantially the same as in this invention in which the apparatus is for drilling wells with a curvature of 200-2000 feet. 
     Combinations of the above described prior art systems may also be used in directional drilling, however none utilize downhole fluid motors having a bent or curved rotor/stator section. Because the rotor/stator section of the motors are typically 2-3 times longer than the bearing section, the prior art systems are not particularly suitable for use in drilling high curvature horizontal wellbores, such as medium-radius (10 to 1,000 feet), from vertical or near vertical wells. The present invention may be used in conjunction with the prior art apparatus such as bent subs, bent housings, and inclined motor shafts. 
     The present invention is distinguished over the prior art in general, and these patents in particular by a system which utilizes a downhole fluid-operated motor having a bent or curved rotor/stator drive section which allows it to be used in high curvature wellbore applications without interfering with the wall of the borehole. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a deviated wellbore drilling system for drilling curved wellbores which have a radius of curvature of from approximately 10 to 1,000 feet relative to a vertical or near vertical wellbore. 
     It is another object of this invention to provide a deviated wellbore drilling system which may be used in high curvature wellbore applications without interfering with the wall of the borehole. 
     Another object of this invention is to provide a deviated wellbore drilling system having a fluid-operated drill motor with a curved or bent drive section connected therebetween for rotating the drill bit independently of the drill string. 
     Another object of this invention is to provide a deviated wellbore drilling system which will reduce the problems associated with utilizing elongate straight fluid-operated drilling motors in directional drilling applications with little loss of power required to rotate the rotary bit at the bottom of the drill string. 
     Another object of this invention is to provide a deviated wellbore drilling system which will aid in obtaining and maintaining control and orientation of the drill bit. 
     A further object of this invention is to provide a deviated wellbore drilling system wherein a curve or bend is located in the drive section of the motor rather than in the drill string above the drive section or in the motor housing below the drive section to more effectively angularly offset the output shaft relative to the drive section axis. 
     A still further object of this invention is to provide a deviated wellbore drilling system which is simple in design, economical to manufacture, and reliable and durable in use. 
     Other objects of the invention will become apparent from time to time throughout the specification and claims as hereinafter related. 
     The above noted objects and other objects of the invention are accomplished by a deviated wellbore drilling system comprising a drill string, a drill bit, and a fluid-operated drill motor having a curved or bent drive section connected therebetween for rotating the drill bit independently of the drill string. The drilling motor has an elongate tubular rotor/stator drive section containing a rubber stator and a steel rotor and the housing is bent or curved intermediate its ends. The motor is preferably of a Moineau configuration. A straight universal section below the bent or curved rotor/stator section contains a flexible connection for converting orbiting motion of the rotor to concentric rotary motion at the bit, and a bearing pack section below the universal section contains radial and thrust bearings to absorb the high loads applied to the bit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side elevation of a conventional, prior-art straight hole fluid-operated drilling motor. 
     FIG. 2 is a schematic side elevation of a conventional drill string system of the prior art utilizing a &#34;bent sub&#34; located in the drill string above a straight fluid-operated motor. 
     FIG. 3 is a schematic side elevation of a conventional drill string system utilizing a &#34;bent motor housing&#34; located below the motor drive section and above the motor bearing pack section for use in directional drilling. 
     FIG. 4 is a schematic side elevation of a conventional drill string system of the prior art utilizing a straight hole fluid-operated drilling motor with the drill bit axis angularly offset from the axis of the motor. 
     FIG. 5 is a schematic side elevation illustrating the problem encountered with conventional drill string systems which utilize a straight hole fluid-operated drilling motor. 
     FIG. 6 is a schematic side elevation of the present system illustrating how a motor having a bent or curved drive section overcomes the problem encountered with the prior art drill string systems. 
     FIG. 7 is a schematic side elevation of the present system illustrating a downhole fluid-operated motor having a single bend in the motor drive section. 
     FIG. 8 is a schematic side elevation of the present system illustrating a downhole fluid-operated motor having multiple bends in the motor drive section. 
     FIG. 9 is a schematic side elevation of the present system illustrating a downhole fluid-operated motor having a curved motor drive section. 
     FIG. 10 is a transverse cross-section taken along lines 10--10 of FIG. 7 showing the rotor/stator section of the fluid-operated motor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The novel features of the present invention can be best understood by a brief description of prior art apparatus commonly used in directional drilling systems. 
     Referring to the drawings by numerals of reference, there is shown in FIG. 1, a conventional fluid-operated drilling motor of the prior art used for straight and slanted hole drilling operations. Conventional motors, such as a Moineau motor comprise three sections, the rotor/stator section which contains a rubber stator and a steel rotor; the universal section which contains the universal joints or flexible connection that convert the orbiting motion of the rotor to the concentric rotary motion of the bit; and the bearing pack section which contains radial and thrust bearings to absorb the high loads applied to the drill bit. 
     In such apparatus a fluid, usually a relatively incompressible liquid, is forced down the stationary drill pipe or drill string and on passing through the fluid-operated motor causes the thereof to rotate the drilling bit. The drill string is held or suspended in such a manner that it does not rotate and therefore may be regarded as stationary. However, it is lowered in the well as drilling proceeds. The forces required to rotate the rotary bit at the bottom of the string are such that in the usual situation the fluid operated motor must be quite lengthy. The rotor/stator section of conventional motors is typically 2-3 times longer than the bearing pack section. 
     Drilling motors of this general character are used in directional drilling, however, because the rotor/stator section of the motors are typically 2-3 times longer than the bearing section, the conventional motor systems are not particularly suitable for use in drilling high curvature horizontal wellbores, such as medium-radius (10 to 1,000 feet), from vertical or near vertical wells. 
     As shown in FIG. 2, some directional drilling systems position a &#34;bent sub&#34; between the top of the fluid-operated motor and the axis of rotation of the bit to the axis of the drill pipe. However, due to the length of the motor required and other structure connecting the rotor of the motor to the bit, the spacing of the &#34;bent sub&#34; from the bit is excessive. Frequently, this distance amounts to approximately 22 feet or more which is objectionable due to the fact that it is difficult to position and to maintain the orientation of the bit in relation to the axis of the drill pipe. 
     Other systems attempt to solve this problem by placing the bend closer to the bit as shown in FIG. 3. These types of systems modify the universal section of the drilling motor. Because the lower end of the rotor in the aforementioned types of motors gyrate about the axis of its stator, some form of universal joint or flexible connection is employed in the driving connection between the rotor and the bit which rotates about a stationary axis. 
     As a clearance must exist between this universal joint or flexible connection and the walls of the surrounding housing to accommodate the flexibility of movement, a bend is formed in the housing of the universal section of the motor. In this manner, the axis of rotation of the bit is angularly related not only to the axis of the drill string but also the axis of the fluid-operated motor. However, the bend is located below the motor drive section and above the motor bearing pack section. 
     Still other systems, such as depicted in FIG. 4, utilize a straight drilling motor with the bit or output shaft angularly offset relative to the longitudinal axis of the drive section. 
     FIG. 5 is a schematic side elevation illustrating the problem encountered with conventional drill string systems which utilize a straight fluid-operated drilling motor in combination with a bent sub or bent universal housing in drilling high curvature wellbores. 
     As shown in FIGS. 6-10, the present invention is distinguished over the prior art by a system which utilizes a downhole fluid-operated motor having a bent or curved rotor/stator drive section which allows it to be used in high curvature wellbore applications without interfering with the wall of the borehole. 
     The present system 10 comprises a downhole fluid-operated motor 11 having a straight universal section 12 which contains the universal joints or flexible connection attached at its lower end to the drill bit 13 and which converts the orbiting motion of the rotor to the concentric rotary motion of the bit. The motor 11 has a straight bearing pack section 14 at the lower end of the universal section which contains radial and thrust bearings to absorb the high loads applied to the drill bit 13. The motor drive section, or rotor/stator section 15 of the motor 11 contains a rubber stator 16 and a steel rotor 17 within the outer housing 18 of the drive section. The motor is preferably of the Moineau configuration but the housing 18 and stator 16 are bent or curved and the rotor 17 is straight. The flexibility of the rotor shaft 17 and compressibility of the rubber stator 16 permit rotation of the rotor without undue binding. 
     FIG. 7 shows one embodiment of the motor 11 wherein the rotor/stator outer housing 18 has a single bend 19. FIG. 8 illustrates a modification of the downhole fluid-operated motor 11 having multiple bends 19 vertically spaced in the outer housing 18 of the rotor/stator drive section. Another modification is shown in FIG. 9 wherein the downhole fluid-operated motor 11 has a curved rotor/stator outer housing 20. 
     A fluid, usually a relatively incompressible liquid, is forced down the stationary drill pipe or drill string and on passing through the fluid-operated motor 11 causes the rotor 17 thereof to rotate the drilling bit 13. The drill string is held or suspended in such a manner that it does not rotate and therefore may be regarded as stationary and the drill string is lowered in the well as drilling proceeds. 
     Although rotor/stator section of the motor 11 is still approximately 2-3 times longer than the bearing pack section, the bent or curved section is located in the long drive section of the motor between the drill pipe and the universal section of the motor. In this manner, the bit or output shaft is angularly offset relative to the drive pipe axis over a sufficient linear distance making it particularly suitable for use in drilling high curvature horizontal wellbores, such as medium-radius (10 to 1,000 feet) from vertical or near vertical wells. 
     Tests conducted on motors according to the present invention which have sharp bends or curved rotor/stator drive sections have demonstrated that they deliver approximately 90  to 95 percent as much power as motors having a straight drive section, therefore loss of power is not a significant problem in drilling curved wells of a radius from 10 to 1000 feet. 
     While this invention has been described fully and completely with special emphasis upon a preferred embodiment, it should be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein.